Basically imagine a week long water fight between 15 million bangkokians and 60 million Thai's and your starting to get an idea!
This is an absolutely amazing attraction and is well worth a trip to Thailand.
Make sure you check out
The Thaistyle Thailand Travel Guide
Equipment Buckets fill or water
Drums / Tubs full of water
Water Balloons / Water Bombs
Water Pistols / Water Guns